Structures of permanent magnet rotating machines can be classified into the radial gap type and the axial gap type. In the radial gap type, a plurality of permanent magnets are arranged in the circumferential direction of a rotor, the poles of the permanent magnets being radially directed, and a stator is arranged so as to face the permanent magnets. Generally the stator has a structure in which coils are wound on an iron core having a plurality of teeth on the surface facing the rotor. By use of an iron core, magnetic flux from the poles of the rotor can efficiently intersect the coils so as to produce a large torque in the case of a motor and a large voltage in the case of an electric generator. On the other hand, there is a problem that use of an iron core generates cogging torque and loss torque based on the hysteresis loss of the iron core to make the initial torque large. In application to a wind generator, for example, slight wind fails to produce voltage due to a large initial torque.
Such a problem can be solved by eliminating the iron core, but this lowers the magnetic efficiency so that large output cannot be obtained by the radial gap type. Thus the axial gap type as shown in FIG. 9 can be designed. In FIG. 9, disk-shaped magnetic bodies (rotor yoke) 102a, 102b are integrally attached to a rotating shaft (shaft) 101 and the rotor yokes 102a, 102b each has a plurality of permanent magnets 103 on the surface. The rotor yokes 102a, 102b are arranged in the direction of the rotating shaft through a spacer 104. While the permanent magnets 103 may be arranged on only one of the rotor yokes 102a and 102b, high magnetic efficiency is provided by arranging the permanent magnets (103a, 103b) on the surfaces of both rotor yokes (102a, 102b). These are called rotors (105a, 105b).
Coils 106 are arranged between the rotor yokes (102a, 102b). The coils 106 are accommodated in a coil base 107 to constitute a stator 108 and fixed to a housing 109. The rotating shaft 101 is rotatably supported by the housing 109 through a bearing 120. This structure can provide large output without an iron core in the stator 108 by making the magnetic pole surface large.
In addition, when Nd—Fe—B type sintered magnets are used, which are strong permanent magnets, the rotating machine obtains high output by fully utilizing their performance, being free from the problem of magnetic saturation of an iron core. Generally a rotor has a plurality of permanent magnets attached on the surfaces of disk-shaped magnetic bodies, using epoxy-based or acrylic-based adhesive for bonding, and the magnetic bodies and the magnets are fixed through only one surface as shown in JP2003-348805 A.
In a large rotor the permanent magnets are subject to large centrifugal force during rotation. Further, the temperature of the rotating machine undergoes a heat cycle from the room temperature to a high temperature according to the operating state. In the case of an Nd—Fe—B type sintered magnet, the surface perpendicular to the direction of magnetization, that is, the surface where the magnet is bonded with the magnetic material disk, has a negative coefficient of thermal expansion (shrinks when the temperature rises, −1.7×10−6 [1/K]). Soft iron is usually used for the magnetic material disk and it has a positive coefficient of thermal expansion (stretches when the temperature rises, 10×10−6 [1/K]). Thus the bonding surface is subject to large stress due to the heat cycle. Consequently, there is a problem that breakage occurs in the adhesive and the permanent magnet comes off due to centrifugal force to lose its function.    Patent Document 1: JP2003-348805 A